1. Field of the Invention
This invention relates generally to a sealing process and an apparatus therefor and, more particularly, to an improved method and apparatus, for effecting a hermetic seal between an electronic component package and a package cover.
2. Description of the Prior Art
Many electronic circuits and components are housed in sealed packages to protect them from possible damage due to exposure to the surrounding environment. Sealants employed in the past include (1) metallic sealants such as solders; (2) ceramics or various glasses; and (3) organic sealants such as epoxies. Of these, only the first two result in a truly hermetic seal since the organic sealants become permeable to large quantities of matter in a relatively short period of time (e.g. days) when compared to the life of the circuit housed within the package. If the sealant became permeable to, for example, water vapor, a chemical reaction could take place with certain ions within the package such as chlorine, sodium and potassium to produce corrosive products which would in turn attach wiring and/or the electronic components, and hasten failure of the packaged circuit.
Several metal alloys have been employed as sealants with varying results. One very desirable property of a metallic sealant is its ability to withstand high temperatures. For example, ordinary lead-tin solder provides satisfactory seals at temperatures up to 125.degree. C. For higher temperatures, such as 150.degree. C., an alloy of 80 percent gold and 20 percent tin is preferable, although it is obviously expensive. Unfortunately, the melting point of this alloy is approximately 280.degree. C., and fusing the material during the sealing process may heat the housed electronic components, possibly resulting in undesirable parameter shifts and premature failure of the circuit within the package.
The danger of circuit or device failure due to high sealing temperatures is even more acute when vitreous sealants are employed, since such sealants have fusing temperatures above 400.degree. C. While vitreous sealants are much less expensive than the previously discussed gold-tin alloy, the higher fusing temperatures have precluded the use of such glass sealants in cases where component heating is a problem.
Methods such as impulse or impact welding using either fixed or rolling electrodes have been developed for sealing packages without unduly heating their contents; i.e. these techniques require metal covers and packages or, at the very least, metal sealing surfaces on both the cover and package. Unfortunately, the equipment needed to carry on such welding techniques is expensive, and special tooling is required for each package size. The metal packages themselves are also relatively expensive.
The term "hot-capping" refers to a method for sealing either metallic or ceramic lids and packages using a metallic sealant. If either the lid or package is ceramic, it must have a metallic sealing surface fused thereto. The seal is effected by applying an electrically heated surface to the cover. However, since the cover must be heated to at least the sealing temperature, the temperature of the electronic component or circuit will be raised.
Since ceramic packages are less expensive than metallic packages, and since vitreous sealing materials are less expensive than metal based sealants, the most economical approach to hermetically sealing packages would be to use ceramic packages and covers and seal them with a vitreous sealant without requiring fused metallization sealing layers.
In the above-identified co-pending application, an improved method and apparatus for hermetically sealing ceramic or metal packages housing electronic circuits is disclosed. Accordingly, a heater preform for sealing an electronic component package to a package cover is positioned between sealing surfaces on the the package and the cover. The heater preform has an area and shape corresponding to that of the sealing surfaces. In addition, the preform has first and second electrical contact areas. A vitreous sealing material is applied between the sealing surfaces and the preform. The heater preform is electrically heated to a predetermined temperature to effect a hermetic seal between the sealing surfaces of the package and the cover, the sealing material and the preform.
Whereas the process disclosed in the above identified copending application proves to a great improvement over known prior art, the need still exists to provide an even more efficient process for effecting a hermetic seal between ceramic packages and package covers at a lower temperature.